Fresh coolant, hoses and thermostat in the spring, when doing timing belts. Last trip to work, temps higher that normal. On the trip home (half an hour) the temps slowly creeped up to a hair under the red, at highway speeds. Fan on as normal.
Slight oil residue noticed in the overflow tank, and that's as far as the diagnosis has gone, as I'm in the middle of re-sheathing and re-shingling my garage.

Initialy I thought of the thermostat being buggered, but that wouldn't explain the oil in the overflow tank. I then thought of head gaskets, but am unclear as to whether overheating would be a symptom of their failure.

So....would a screwed waterpump introduce oil into the cooling system, and allow the system to overheat?

Its a head gasket-both the oil in the coolant tank and overheating are classic signs.
You can buy a Athena top end gasket set for $100 at bike bandit.It includes the valve seals also.Do both head gaskets-and the valve seals.Any questions-post back-alot of us have done this project.Regards,Johnyy

You don`t say whether you have found antifreeze or white / tan foam in the oil. Suggest looking there next. Also, I believe antifreeze burns white, that would show at the exhaust tip, and if there`s sufficient antifreeze leaking into the oil, i would think it would thin the oil enough to reduce cooling efficiency. And a compression check may be revealing.

Best get the cooling system pressure tested as a first step before you start ripping the heads off.
I did have a problem with the weep hole being blocked off by a previuos owner as the mechanical seal on the pump was leaking. So water went into the oil and is surprising how little water in the oil makes stange whitish blue smoke.
The water pump front bearing tends to go west which then causes damage to the mechanical seal .If the mechanical seal in the water pump is defective then the whole pump should be changed-the impellor will probably have been fouling the casing and damaged. The early water pumps had a plastic impellor which is notorious for failure to circulate water if damaged casusing overheating. Its a simple job the replace the water pump mwith the better quality one witha metal impellor. At least if you sort this out its a less expensive option than tearing the heads off before any further causal analysis.

Whitish smoke out of the right side, and insane oil consumtion. Originally thought the oil consumtion was oil seals, and it still may be part of the issue, but since the oil pressure is higher that cooling system pressure, a failed O-ring pair @ the head and/or head gasket is looking more and more likely.
My compression tester doesn't have a 12mm thread! Bugger.

Bought a parts washer on Friday, and will finish the garage roof today, so my attentions can turn back to the bike.
Honestly, after 30 years, I have no issues with cleaning the crap off of valves, piston tops and combustion chambers, lapping the valves in, getting the heads skimmed so they're dead flat, and torquing things back together with new oil seals.
To me, the bike has always sounded a bit off....maybe the gasket has been pooched all along. With the oil usage I've had, I'm expecting a LOT of crap on the valves.

No drips near the water pump. I'll have a look for a blocked weep hole.

Steve,
Insane Oil consumption sounds like you have a problem with the bores but you mention White smoke and not blue smoke ?????.
Oil can be sucked into the combustion chamber via the inlet valve oil stem particularly when there is maximum vacuum on then overrun causing blue smoke. White smoke normally means water in the combustion chamber and if you have anti freeze in the coolant then a sweet smell should accompany the white smoke. Black smoke is an overrich petrol /air mixture.
Ride the bike and get someone to follow you to check the smoke as its difficult to see when you are riding the bike. Gun the bike and then throttle off before gunning it again. if you get smoke when gunning it again then it sounds like valve seals as the oil will go into the combustion chamber on the overrun and when you open it up again the oil will burn. if you get smoke all the time it will be the bores/pistion rings. I have a pressure gauge with a rubber seal that goes over the sparking plug hole and it also has a non retun valve in it so I can see the maximum pressure gained. I test the compression by FULLY opening then throttle and cranking the engine with the kill switch off and all plugs removed. If the pressure is above 145 PSI at sea level then thats acceptable. if the pressure is low and also the difference in pressure is noticible then check the pressure again with a spoon full oil in the combustion chamber via the sparking plug hole to seal the rings .If the pressure is markedly higher then the rings/bores are worn and its a big strip down. If you in fact find the oil seals on the valves are gone then they can be replaced without removing the cylinder head by using a specialist valve spring compressor availble on E bay.I have done this before by following a wonderfull thread on this website that explains the procedure very well indeed. Good Luck

Well.
Pulled the right side head, and found sparkling clean piston tops. Hmm. Too clean.....steam cleaned, I'd say. Also very heavy carbon build-up in the exhaust ports...like chunks.

No discerable ridge at the top of the bore. GOOD
A very strangely distorted larger-of-the-two O-ring on the oil restictor. Bad
All guides test tight. GOOD
Heavy carbon build up in exhaust. Bad
Some evidence of coolant passing in combustion chambers. Bad.
Valves look good, after clean up. GOOD
Oh, and when I started, oil in the overflow tank, and blech on the rad cap. Bad.

Strange that? If water has been getting into the combustion chamber its normally via defective head gasket which can be seen when removed. If no damage to the gasket I would check the cylinder head for warping. Also where the cylinder head bolts tigh or did they loosen off too easily???
The distortion of the second rubber 0 ring on the rocker oil feed restrictor seems odd as well- its really a "belt and braces "method of sealing the oil feed off. The head gasket also assists this. When you put the new gasket on make sure its a genuine Honda one as there are some shoddy alternatives around and make sure ALL traces of the old gasket are removed from the mating surfaces and torque the head down carefully to the maintenance manual settings and do not get mixed up with the differences between stated torque settings standards.
If you do not have a torque wrench and buy/borrow a torque wrench test it on a nut and bolt in the vice before attacking the head. This will give you a feel for its use as there is nothing worse than stripped cylinder head bolts!!!
Good luck

It dosent take much of a leak to get anti freeze in the combustion chamber.Once you remove the gasket-the leak may be undiscernable-but it is there.Remember it is under alot of pressure-all it needs is a tiny hole.I think if you finish up the head gasket job-it will run like a pro. And when you first start it up-it will smoke like the devil for 5 minutes or so-its just burning off residue-it will quit.Be carefull about the oil 0-ring placement on the head when you reassemble-it is critcal that it is correct. Regards,Johnyy

If they stay in the block-leave them there-polish them up with some 0000 steel wool, wipe off ,wipe on oil. If they are in the head-wd 40 them to an inch of there lives, let them sit for a few hours- tap them lightly, they sould come out.Regards, Johnyy

Has it overheated? If not,chances are it is not warped. Even head gaskets degrade over time. You can take it in and have the heads machined, I think is costs about $100 each.
I just removed ALL of the gasket residue from the head and block. You will see some staining from gasket/heat in the aluminum.As long as the head is not warped, disregard the staining. Just be sure ALL gasket residue is removed.Use a gasket scrapper.you can tap out the block dowels to make it more simple.
The dowles have a rounded edge that fit into the block-watch for that Regards, Johnyy